1. Field of the Invention
The present invention relates to an encoding/decoding apparatus which can accomplish a high efficient encoding of input information data.
2. Related Background Art
In association with the digitization of an image signal, a high efficient encoding technique is also important.
There is an orthogonal transformation encoding as effective means for high efficient encoding. According to the orthogonal transformation, an input time-sequential signal is transformed into an orthogonal component (for example, frequency component).
As specific examples of the orthogonal transformation, a Fourier transformation, a discrete cosine transformation (hereinafter, abbreviated to DCT), a Hadamard transformation, and the like are known. In particular, the DCT is considered as an orthogonal transformation suitable for image information.
An encoding method using the DCT will now be described.
FIG. 1 is a block diagram of a conventional encoding apparatus using the DCT.
In FIG. 1, a digital image signal which is input from an input section 1 is divided into blocks on a DCT unit basis by a block forming circuit 2. As a size of each of the divided blocks, a 2-dimensional DCT block of total 64 pixels comprising [(eight pixels in the horizontal direction).times.(eight pixels in the vertical direction)] is frequently used.
The block-formed image signal is two-dimensionally DCT transformed by a DCT circuit 3 into a DCT component.
The transformed DCT component is quantized by a quantization circuit 4 and is variable length encoded by a variable length encoding circuit 5. The encoded data is converted to a data of a predetermined rate by a buffer memory 6 and is output therefrom.
In recent encoding apparatus using the DCT, an apparatus for efficiently encoding by using an inter-field correlation has been proposed.
Namely, when the inter-field correlation is weak (low), the intra-field DCT (a DCT process is executed in the block comprising 4.times.8 pixels) is executed (refer to FIG. 2B) (hereinafter, referred to as a field transforming mode) and, when the inter-field correlation is strong (high), the intra-frame DCT (a DCT process is executed in the block comprising 8.times.8 pixels) is executed (hereinafter, referred to as a frame transforming mode), thereby raising an encoding efficiency (refer to FIG. 2A).
A conventional selecting method of selecting a transforming mode will now be described hereinbelow.
Differential absolute values between the even field pixels of the block (8.times.8 pixels) which is read out from the block forming circuit 2 and the odd field pixels which are one field after the even field pixels are calculated, the intra-block total of those differential absolute values is calculated, and its total value is compared with a predetermined threshold value, thereby selecting the transforming mode in accordance with the result of the comparison (namely, a field correlation is detected).
In the above judgment, a selection regarding whether the signal can be concentrated into a lower frequency region is made by executing the transforming process the selected modes (frame/field). That is namely, in case of encoding, a selection regarding whether an amount of codes which are generated is smaller by executing the selected transforming process becomes a problem.
Therefore, a fairly large circuit is needed to accurately select the proper transforming mode. In addition, since it takes a predetermined processing time until the transforming mode is selected, an inconvenience occurs in case of a moving image which needs a real-time process.
In the conventional transforming mode selecting method as mentioned above, for instance, in the case where there is a correlation of every two lines in the data in the block, a large difference occurs when comparing FIGS. 3 and 4A.
Namely, when the transforming mode is selected by the conventional method as mentioned above, in FIG. 3, it is judged that the inter-field correlation is high, so that the frame transforming mode is selected. In FIG. 4A, it is judged that the inter-field correlation is low, so that the field transforming mode is selected.
When the DCT process is executed for the block data shown in FIG. 4A, the data in the block is rearranged as shown in FIG. 4B. The block data is divided into an even field pixel data block and an odd field pixel data block. The DCT process is executed for each block. In this case, therefore, as transformation coefficients of the even/odd field pixel data block, a coefficient also exists in the high frequency region, so that an inconvenience such that an encoding efficiency is contrarily deteriorated occurs.